1. Field of the Invention
The present invention relates to illuminating apparatuses that illuminate illumination areas of objects.
2. Description of the Related Art
To date, illuminating apparatuses for illuminating an object with light beams emitted from light sources have been used in image pickup apparatuses such as video cameras and digital cameras when the object side is in low light. Such illuminating apparatuses include a light source and optical components such as reflecting mirrors and a Fresnel lens that can efficiently guide light beams emitted from the light source forward (to the object side).
Among such illuminating apparatuses, some illuminating apparatuses can efficiently collect light beams emitted from a light source in many directions into a required illumination field angle.
Moreover, some illuminating apparatuses include optical members using total reflection such as a prism light guide instead of a Fresnel lens disposed in front of a light source so as to improve collection efficiency and reduce the size of the apparatuses.
Among these, an illuminating apparatus that illuminates an object with light beams emitted forward from a light source via a small prism having a high collection efficiency is well known (see Japanese Patent Laid-Open No. 4-138438).
The illuminating apparatus disclosed in Japanese Patent Laid-Open No. 4-138438 can markedly increase the illuminance in the vicinity of the center of the object.
Moreover, some illuminating apparatuses have an illumination optical system reduced in size by disposing a prism in the anterior position to a light source. Furthermore, some illuminating apparatuses have a light-emergent surface that receives and emits light beams perfectly reflected from a prism and is inclined with respect to an optical axis.
Among these, an illuminating apparatus having an optical member formed of a light guide located in the vicinity of a light source is well known (see Japanese Patent Laid-Open No. 8-234277). The illuminating apparatus collects light beams emitted from the light source and converts the light beams into uniformly distributed light beams using the optical member so as to illuminate a surface with a small optical loss.
The illuminating apparatus disclosed in Japanese Patent Laid-Open No. 8-234277 collects light beams in a longitudinal direction of the light source by repeating total reflection inside the light guide so as to uniformly illuminate the surface with a small optical loss.
In addition, an illuminating apparatus reduced in size by disposing a prism having an asymmetrical shape with respect to an axis of illuminating light in front of a light source is also well known (see Japanese Patent No. 3685516).
In general, when image pickup apparatuses such as cameras are used, illumination is often performed over a relatively wide range at the object side. Therefore, the directivity of illuminating beams is not carefully considered. However, expressiveness of acquired image information of objects widely differs according to differences between illuminating conditions. For example, the surface state of a specific object to be captured can be emphasized by controlling incident angles of illuminating beams with respect to the object. Moreover, in order to capture momentary images of a moving object without image blurring, an effective illumination with a short-time emission of a few to dozens of microseconds such as a strobe light is required.
In general, electronic flash devices are used as auxiliary light sources in image pickup apparatus such as cameras. In these electronic flash devices, the required light power for one image capture has been increasing. In particular, more electric power is required for illumination when images of a dark object are captured. Under such conditions, more light is required to capture images of an object located at the same distance at the same brightness. Moreover, smaller illuminating apparatuses have become required as the size of image pickup apparatuses have become smaller.
Cylindrical light-emitting sources, in particular, xenon light-emitting tubes are advantageous as light sources for high-intensity illumination. However, light-emitting tubes typified by xenon light-emitting tubes have various light power distributions in accordance with emission timing.
In a light power distribution of a light-emitting tube, a portion having a high intensity exists along the inner wall of the light-emitting tube, and the center of light emission is changed in accordance with emission timing.
Hereinafter, the light-emergent surface at this moment is referred to as a cylindrical light-emergent surface of a cylindrical light-emergent source since the vicinity of the inner wall of the light-emitting tube mainly emits light.
FIG. 9 illustrates light power distributions in a section of a light-emitting tube, the positions of portions with high light intensity being changed in accordance with emission timings t1, t2, and t3. At the timing t1, the center of light emission lies on a portion 121 on a light-emitting tube (inner wall of the light-emitting tube) 120. The center of light emission moves to a portion 122 at the timing t2, and moves to a portion 123 at the timing t3. When the center of light emission moves in this manner, distribution of illuminating light on an object is changed accordingly. In particular, when an object is illuminated with light beams having a directivity oblique to the object, illuminance distribution is significantly changed.
In the illuminating apparatus disclosed in Japanese Patent No. 3685516, for example, a hatched portion 102 shown in FIG. 10 is the area on which light beams passing through a refracting lens 100 and light beams reflected from reflecting mirrors 101a and 101b at either side of the refracting lens 100 are incident at the same time. Light paths shown in FIG. 10 are obtained by tracing light beams emitted from the center 103a of a light-emitting tube 103. Light beams emitted from the back surface of the light-emitting tube 103 and reflected from the reflecting mirrors 101a and 101b are shown by dotted lines. As is clear from FIG. 10, areas illuminated by these light beams at the same time are not superposed. Moreover, light paths of light beams emitted from a light-emitting point located on the front surface of the light-emitting tube 103 also differ from those shown in FIG. 10. In this manner, each light-emitting point has an individual illuminating range. Therefore, when the position of the light-emitting point of the light-emitting tube 103 fluctuates, the distribution of illuminating light is widely changed in accordance with the position of the light-emitting point. Accordingly, it becomes difficult to illuminate the object efficiently and uniformly.